Work on style, add some sentences to blurb, explain that 10468 is more
general than had been described, etc etc

When I merged the fix for #7351, and implemented proposal 214 (4-byte
circuit IDs), I forgot to add a changes file.  Later, we never noticed
that it didn't have one.

Resolves ticket #11555.  Thanks to cypherpunks for noticing this was
missing.

This is a cherry-pick of 75e10f58 into
the master branch.

Also, add a script to do this, since doing it manually with fmt sucks.

(Not taking bug11235 since it's a whitespace fix)

(users want to know about client fixes before they want to know about
unit test fixes.)

also clarify a few explanations. so far so good!

Still haven't edited any.

Also, remove all the changes/* entries that got merged back in
6b5ef388.

Coverity spotted these.

This collects all the changes file, and find the stuff that also
appeared new in 0.2.4.21.

also replace them with the versions of the changelog stanzas from
release-0.2.4, for consistency